uu.seUppsala University Publications
Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
On the Optimization of Point Absorber Buoys
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
Show others and affiliations
(English)Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-218218OAI: oai:DiVA.org:uu-218218DiVA: diva2:695080
Available from: 2014-02-10 Created: 2014-02-10 Last updated: 2017-08-24Bibliographically approved
In thesis
1. Grid Connected Three-Level Converters: Studies for Wave Energy Conversion
Open this publication in new window or tab >>Grid Connected Three-Level Converters: Studies for Wave Energy Conversion
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents an electrical system analysis of a wave energy converter (WEC) for the objective of grid connection. To transfer the enormous amount of power from waves to the load centers, efficient power electronic systems are essential. This thesis includes the modeling of a buoy–translator dynamics and the modeling of a linear permanent magnet generator along with simulation and experimental validation. Diode bridge rectifiers are considered for rectification to avoid the complex linear generator control at the input side. To reduce the size and the cost of energy storage elements, DC voltage regulation is done using a DC/DC converter.

To achieve smooth and high power, many WECs need to be connected to a common DC link. A neutral point clamped inverter is considered for the DC/AC conversion due to its advantages over conventional topologies. Various pulse width modulation schemes are tested for the inverter to choose the optimum PWM method. The harmonics in the inverter output voltage is derived numerically and compared with simulation and experiment to understand the effect of dead-time in the inverter operation.

Depending on the load current drawn from the inverter, the voltages in the two input capacitors of a three-level neutral point clamped inverter deviates from equilibrium unless the neutral point is grounded. To avoid this voltage imbalance as well as to regulate the DC link voltage a dual output boost converter with pulse delay control is proposed. The modeling, simulation and experiments show an improvement in the compensation voltage using pulse delay control compared to the previously proposed methods in the literature. The synchronous current control and the grid connection of the three-level converter have been accomplished in the laboratory. 

Finally, the three-level power converter system has been tested with a linear permanent magnet generator at Lysekil to analyze the controller requirements.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 78 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1123
Keyword
Wave energy, power converters, control strategies, grid connection
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-218219 (URN)978-91-554-8875-8 (ISBN)
Public defence
2014-03-26, Häggsalen, Lägerhyddsvägen 1, Angstrom laboratory, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2014-03-04 Created: 2014-02-10 Last updated: 2014-12-02Bibliographically approved
2. Wave Loads and Peak Forces on Moored Wave Energy Devices in Tsunamis and Extreme Waves
Open this publication in new window or tab >>Wave Loads and Peak Forces on Moored Wave Energy Devices in Tsunamis and Extreme Waves
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Surface gravity waves carry enormous amounts of energy over our oceans, and if their energy could be harvested to generate electricity, it could make a significant contribution to the worlds power demand. But the survivability of wave energy devices in harsh operating conditions has proven challenging, and for wave energy to be a possibility, peak forces during storms and extreme waves must be studied and the devices behaviour understood. Although the wave power industry has benefited from research and development in traditional offshore industries, there are important differences. Traditional offshore structures are designed to minimize power absorption and to have small motion response, while wave power devices are designed to maximize power absorption and to have a high motion response. This increase the difficulty of the already challenging survivability issue. Further, nonlinear effects such as turbulence and overtopping can not be neglected in harsh operating conditions. In contrast to traditional offshore structures, it is also important to correctly account for the power take off system in a wave energy converter (WEC), as it is strongly coupled to the devices behaviour.

The focus in this thesis is the wave loads and the peak forces that occur when a WEC with a limited stroke length is operated in waves higher than the maximum stroke length. The studied WEC is developed at Uppsala University, Sweden, and consists of a linear generator at the seabed that is directly driven by a surface buoy. A fully nonlinear CFD model is developed in the finite volume software OpenFOAM, and validated with physical wave tank experiments. It is then used to study the motion and the forces on the WEC in extreme waves; high regular waves and during tsunami events, and how the WECs behaviour is influenced by different generator parameters, such as generator damping, friction and the length of the connection line. Further, physical experiments are performed on full scale linear generators, measuring the total speed dependent damping force that can be expected for different loads. The OpenFOAM model is used to study how the measured generator behaviour affects the force in the connection line.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 86 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1551
Keyword
OpenFOAM, CFD, Wave power, Tsunami waves, Extreme waves, Offshore
National Category
Marine Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-328499 (URN)978-91-513-0054-2 (ISBN)
Public defence
2017-10-20, Polhemsalen, 10134, Ångström, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2017-09-28 Created: 2017-08-24 Last updated: 2017-10-17

Open Access in DiVA

No full text

Authority records BETA

Sjökvist, LinneaKrishna, RemyaRahm, MagnusCastellucci, ValeriaHagnestål, Anders

Search in DiVA

By author/editor
Sjökvist, LinneaKrishna, RemyaRahm, MagnusCastellucci, ValeriaHagnestål, Anders
By organisation
Department of Engineering Sciences
Engineering and Technology

Search outside of DiVA

GoogleGoogle Scholar

urn-nbn

Altmetric score

urn-nbn
Total: 549 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf